Page 72 - Analysis and Design of Energy Geostructures

42 Analysis and Design of Energy Geostructures

2.4.6 The seasonal factor of performance
The efficiency of ground source heat pump systems along seasons can be characterised
through the seasonal factor of performance, SFP. This factor includes not only the
energy of the heat pump but also that of other energy-consuming elements (e.g. circu-
lation pumps) (Brandl, 2006). The SFP is defined as

Energy output usable from the energy system ½kWhŠ
SFP 5 ð2:2Þ
Energy input of the energy system ½kWhŠ
Values of SFP of 3.8 4.3 can be achieved with standard electric heat pumps, with
an increase of 10% 15% when special devices with direct vaporisation are used
(Brandl, 2006). The seasonal performance of ground source heat pump systems is gen-
erally represented in a Sankey diagram that graphically indicates the energy budget of
a system with the internal and external energy fluxes.

2.4.7 Possible applications of ground source heat pump systems

Two main types of application of ground source heat pump systems can be foreseen
depending on the features of the given location and project, and the number of
energy geostructures required:
1. Monovalent systems relying only on energy geostructures to provide the entire
amount of heating and cooling needs. This type of system is rare but achievable
under certain conditions (i.e. significant groundwater flow and favourable condi-
tioning loads). In these systems at least 70% of the extracted energy may be injected
when encountering minimum groundwater flow, although injecting more than
90% of the extracted energy may compromise the long-term efficiency of free cool-
ing (SIA-D0190, 2005). An example of such a system is the industrial building
Lidwil at Altendorf, Switzerland. The system uses 120 spun energy piles out of 155
spun piles constituting the entire building foundation that are equipped with two
U-shaped pipes per pile (embedded within a gravel layer characterised by a ground-
water velocity between 100 and 150 m/day), and provides 160 kW of heating using
three heat pumps of 18 kW each with a COP of 2.9 3(SIA-D0190, 2005).
2. Bivalent systems using energy geostructures to provide only a proportion of the
heating and cooling needs, the rest being satisfied using conventional heat sources.
An example of such a system is the Dock Midfield at Zürich airport, Zürich,
Switzerland. The system uses more than 300 energy piles out of 440 piles to pro-
vide 65% of the heating needs and 70% of the cooling needs. The remaining pro-
portions come from district heating for heating and from a cooling tower for
cooling, respectively (Pahud and Hubbuch, 2007a).